1. Field of the Invention
The present invention provides improved high molecular weight, polyethylene terephthalate (PET) compositions, and methods of increasing the molecular weight of PET polymers.
2. Description of the Prior Art
Polyethylene terephthalate (PET) is one of the most commercially important resins used for fibers, film and plastics applications. It is usually prepared by the polycondensation of ethylene glycol with dimethyl terephthalate or terephthalic acid at elevated temperatures with the removal of methanol (or water) and ethylene glycol as by-products. The intrinsic viscosity of the polymer obtained in such a melt condensation process normally is between 0.5-0.7 which is adequate for many applications. However, in many other applications in which high strength and toughness are required in the final fabricated product, e.g., in industrial yarns, blow molded, extruded or thermoformed products, high molecular weight PET with an intrinsic viscosity above 0.9 is desired. In the normal PET polymerization reactors it is very difficult to obtain high molecular weight polymer since the reaction rate becomes slow and diffusion controlled in the final stages. Prolonged heating causes the degradation of terminal hydroxyl groups leading to the formation of undesired carboxyl end groups which promote further degradation (hydrolytic or thermal) of the polymer.
To overcome these limitations, post-polymerization of low molecular weight PET in the solid state is currently practiced to some extent. However, the solid state polymerization process is slow and requires special equipment for large scale operation.
An alternative approach would be to chain extend PET in the melt-phase with suitable "chain extenders" which would react readily with the hydroxyl and/or carboxyl end groups of the polymer, such coupling reactions leading to increased molecular weights.
In principle, these chain extenders must preferably be bifunctional, thermally stable and capable of reacting quickly with the polyester chain ends, via the nearly irreversible addition or ring-opening type reactions which do not evolve small molecule by-products.
Although compounds such as diphenyl carbonate and diphenyl oxalate were proposed as chain extenders for PET by T. Shima. T. Urasaki and I. Oka. in Adv. Chem. Ser. 128. 183 (1973), these reagents form high boiling by-products such as phenol and ethylene carbonate which are difficult to remove.
Some addition-type chain extending agents such as bisepoxides dianhydrides and diisocyanates have also been disclosed in U.S. Pat. No. 3,553,157 to Dijkstra et al., but these reagents lead to the formation of some undesirable branching and/or thermally unstable linkages in the polymer.
For example, epoxides generate hydroxyl groups which can participate in chain branching/gelation reactions, while isocyanates can lead to the thermally unstable urethane linkages.
Other addition-type chain extenders that have been reported recently for the chain extension of PET include bisoxazolines and bis (1,3 oxazine-4-ones), as reported by H Inata and S. Matsumura in the J. Appl. Polym. Sci., volumes 30 3325 1985) and 32, 4581 (986), respectively. These reagents appear to require long reaction times in a reactor and their efficiency in an extruder process, which is an economically preferred process, is unknown.
In addition, stabilized polyester molding compositions are disclosed in U.S. Pat. No. 4,499,219 to Buxbaum et al. which include a polyepoxide as a first stabilizer and a second stabilizer which includes certain bisoxazolines and/or certain dicarboxylic acid imides and amides. Such Patent states that when processing the compositions, only a slight fall in viscosity or none at all is observed.
It would be desirable to provide polyethylene terephthalate resins which exhibit an increased viscosity without the need for post-polymerization techniques and without forming unwanted by-products.